Thermoplastic molding is used to create a wide variety of useful articles. In general, the process of thermoplastic molding involves heating a thermoplastic material to its glass transition temperature, at which point the material becomes pliable. Other steps in the process include shaping the pliable thermoplastic into the shape of a desired article and allowing the article to cool. Once a thermoplastic material cools to a temperature beneath the range of its glass transition temperature the material becomes significantly less pliable and maintains its new shape. A number of processes have been developed for shaping thermoplastics including single and twin sheet thermoforming.
The twin sheet thermoplastic molding process is often used to create articles that have a hollow region formed between thermoplastic sheets joined to one another at their edges. Examples of articles that have a hollow space between joined thermoplastic sheets include serving pans and thermos bottles. See, for example, U.S. Pat. No. 5,427,732 to Shuert, which is herein incorporated by reference in its entirety. While such articles are clearly useful, most hollow articles are not suitable for use in load-bearing applications.
Thermoplastics are used to laminate various articles including some load-bearing structures. These methods often involve applying thermoplastic sheets to a preformed rigid structure. See, for example, U.S. Pat. No. 5,833,796 to Matich, which is herein incorporated by reference in its entirety. The structural component is essentially rigid and a thermoplastic skin is applied to either one or both sides of the structural component.
The manufacture of articles by twin sheet thermoplastic molding often involves the use of complimentary male and female molding tools. In one common methodology a thin sheet of thermoplastic material is heated until it is pliable, and positioned adjacent to a male mold. The thermoplastic sheet is then moved relative to the tool's surface until the sheet assumes the same shape as the surface of the tool.
Similarly, a second sheet of thermoplastic material is heated until it becomes pliable. The heated second sheet is then centered over the cavity of a female molding tool and moved relative to the female tool molding until the interior portion of the second sheet substantially conforms to the interior shape of the female tool.
One variation of this method, sometimes referred to as vacuum-assist molding, uses vacuum to help draw heated thermoplastic sheets into contact with the surface of the tools. Irrespective of how they are formed, after the two thermoplastic sheets have taken on the shapes of the male and female molds, the edges of the sheets are pressed together and welded to form a single article. For a further discussion of vacuum-assist thermoplastic molding, see, for example, U.S. Pat. No. 5,641,524 to Rush et al., which is hereby incorporated by reference in its entirety.
Vacuum-assist molding works well to manufacture articles that have convex or shallow concave features. The method does not work as well to manufacture articles that have deep narrow concave features. If the female mold cavity is too deep or the sides of the mold too steep, the attractive force supplied by vacuum alone may be insufficient to properly seat the sheet on the interior surface of the female mold.
One method developed to address this problem is referred to as plug-assist molding. Briefly, in plug-assist molding, a rigid tool is used to push a heated sheet at least partly into the cavity of a second tool with a surface shape complimentary to the shape of the first tool. It is easier to manufacture articles that have deep narrow features using plug-assist molding, than it is to manufacture these types of articles using vacuum-assist molding. For an additional discussion of plug-assist molding, see, for example, U.S. Pat. Nos. 6,379,606 to Chun et al., and 5,641,524 to Rush et al., both of which are hereby incorporated by reference in their entirety.
Plug-assist molding works well, but it too has some limitations. The use of the plug-assist molding method is problematic if the goal is to produce articles with load-bearing capabilities. Problems arise because plug-assist molding tends to produce articles that have an uneven distribution of thermoplastic material across the surface of the article. This occurs because the edges of the thermoplastic sheet are clamped in place while the plug contacts the interior of the sheet. As the plug advances material gathers on the leading face of the plug, stretching and thinning the portion of the sheet between the clamped edges and the leading face of the plug. As a result, articles formed by simple plug-assist may have relatively thick edges, bottoms and tops, and relatively thin sides.
An uneven distribution of material resulting in an article with thin sides is especially problematic if the article is used in a load-bearing capacity. The side sections join the top and bottom of the article, and in many load-bearing articles the sides support the top. In these applications the sides bear most of the load and since the sides are thinned in plug-assist molding, articles produced using this technique may be weaker than expected given the composition and thickness of the starting materials.
One way to correct problems caused by differential thinning is to begin the process by using thicker thermoplastic sheets. Another approach is to add additional thermoplastic material to specific portions of the article while it is being formed. See, for example, U.S. Pat. No. 5,885,691 to Breezer et al., which is hereby incorporated by reference in its entirety. Other methods for addressing this problem have also been advanced. See, for example, U.S. Pat. No. 5,427,732 to Shuert, which is hereby incorporated by reference in its entirety.
Given the limitations of currently used methods for making load-bearing articles from thermoplastics and the myriad of potential uses for load-bearing thermoplastics articles, there is a need for additional methods and machinery for the manufacture of such articles.
One area in need for strong, lightweight, load-bearing articles is the manufacture of pallets. Most pallets are designed for use with standard forklift trucks and crane lift cradles and are used throughout the world for the storage and transportation of goods. The majority of pallets in use worldwide are manufactured primarily from wood.
Wood is easy to work with, inexpensive, and well known in the warehousing and transportation industries. These properties account for the widespread use of wood in the manufacture of pallets. However, the use of wood in the manufacture of pallets does have some drawbacks. For example, wooden pallets absorb moisture, rot, splinter, and may harbor pests. The propensity of wooden pallets to harbor pests means that wooden pallets may inadvertently transport harmful fungi, bacteria, and insects between different eco-systems. The introduction of pests into a local eco-system via wooden packing materials, including wooden pallets, can have devastating effects on the local ecosystem.
Given the problems associated with wood-based pallets, there is a real need for pallets made from inexpensive, easy to manufacture, stable, and biologically inert materials, such as plastics. Examples of pallets made at least in part from plastics, can be found in U.S. Pat. Nos. 3,915,098 to Nania, and 6,216,608 to Woods et al. and in U.S. Patent Application Publication Number 2002/0112653 A1 to Moore, et al. All of the aforementioned references are herein incorporated by reference in their entirety. Despite the existence of these and other plastic-based pallets the continued widespread use of wooden pallets demonstrates the need for additional plastic based pallets.
Clearly, there is a need for load-bearing pallets constructed of stable, inexpensive, easily formed, and relatively inert materials, as well as methods and apparatus for manufacturing such pallets.